In the past, chart recorders have been used for a variety of applications to provide an accurate time history of a particular measured parameter. Each reading is indicated by a mark on a moving chart driven by a constant speed motor. The time between readings is proportional to the distance between marks on the chart. Existing chart recorders provide an accurate time history by driving a constant speed motor with a readily available AC line voltage.
It is apparent, however, that there is often a need for a portable battery operated chart recorder that can provide a continuous time history of a measured parameter especially an environmental type parameter such as temperature, pH, conductivity, dissolved O.sub.2 or turbidity of a body of water, or air pollutants at unattended field locations where AC line voltage may be unavailable.
A portable chart recorder may be driven by a mechanical drive including a spring wound motor with an escapement. In recorders of this kind, the chart motion is controlled by the escapement. However, as the spring runs down, the accuracy of the time record varies.
Existing portable battery operated chart recorders that use constant speed motors work well as long as the speed of the motor remains constant. However, for a battery operated device, maintaining a correct time readout has proved difficult and expensive because as the battery dissipates, the motor speed varies. This speed variance is particularly troublesome because it is cumulative. If each reading is off 1 minute every hour, at the end of a day the inaccuracy has grown to 24 minutes.
Speed controls such as governors can be used to regulate the speed of a DC motor but these devices are complicated and expensive and have the further disadvantage of consuming large amounts of current and therefore reducing battery life. Also in the past, voltage control circuits have been employed to deliver a constant voltage to the DC motor to provide a constant speed. However, experience has shown that it is very difficult to maintain a constant voltage from a DC battery. If the system is used in a field location, exposed to the elements, changes in temperature and other ambient parameters may have significant effects upon the voltage control circuits and the battery itself.